Arc process for the production of fume



1952 c. SHEER ETAL 2,616,842

ARC PROCESS FOR THE PRODUCTION OF FUME Filed Jan. 13, 1951 G AS ELEcTRosTA-r'm PREC\PITATOR FUMI:

To Baas o. a, ZNVENTOR.

CHARLEs suaa BY SAM EL K MAN Patented Nov. 4, 1952 CharIes'Sheer;

N 'ew'. York, andi SamuetKormam. Brooklyn NaYt Application January13,'1951j Seri alNo: 205,936"

This" is" a processfor the production are refractory substance in veryfinely divided form commonly known as a fume. Practicaiusesfor theprocess include the reduction of cement ingredients to extremely finelydivided'formor a to reduce. a substancesuch as silica to a fume form.This latter substance Will be used herein as" the substance to beconverted as a specific example. It is possible" by the process to produce such materials havingparticle sizes ranging' from 10 to I00milli-microns in diameter;

This processdepends upon the peculiar property of 'a higherosion arc, ofvaporizing and ionizing refractory matter such as siliceous mattercontained in the electrode of' the arc, even though such material isextremely refractory. The process depends upon the fact also that ifthis vaporized silica is rapidly condensed and collected withoutsubjecting it to chemical combination, it may be recovered in theformof'the fume described;

This case is a companion case to the following other applications of thesame applicants: Ser'. No. 765,148; filed July 31; 1947, for Process ofReducingRefractory Ores, and Ser. No. 47,425; filed September 2, 1948',for Process of Treating Ores:

Silica fume is the namecommercially applied to very finely dividedsilica, or silicon dioxide. In chemical composition it is identical withordinary White sand, which furnishes the main ,raw material for itsproduction, but in physical form it isfundamentally different. Acharacteristic feature of the fume is its minute particle size; but

it probably differs also from sand in its physical properties; i

One principal commercial use of silica fume-is as" a compounding.ingredient in theproduction o1" rubben. In the manufactureof rubber,silica fume isused'as a reinforcing agent. When inproperphysicalconditionwhich this process produces, the fume yields asuperior grade of rubber; -It-has also" the. distinct'advantage that itpermitsth'e manufacture: of rubber tires orsimi- Ian articlesi'n white,orany' color" desired, because are between such electrodes to produce aJet of vaporized material, under conditions which will cause thefume tocondense without chemical alteration-L This inventionaccordinglycomprisesa method 7 comprising the advantages-andaccomplishing results and involving the relationship of the" steps onetoanother, which will'be exemplified in the method herein described and'the scope 'of" the appl'ication-ofthe-invention will be indicated in theclaims.

For afuller understanding of the nature and objects of the invention,reference should behad to the following detailed description taken inconnection with theaccompanying-drawing, in which the figure is anelevation of the apparates to produce the-fume.

The high erosion are here referred to and employed in thisprocessis ofthe' same general nature as the "high intensity are as the term is usedin the literature" followingthe nomenclature introduced byB'assett andSperry. The are was developed by them forums in search lights, and theygave it its name because of the high intensityofillumination thatresults as soon as the transition point is passed. In the lightingindustry however, the erosion of the electrodes was-a dead loss, and theintensity'of light only was desired. The erosion was therefore kept tothe minimum. Theh'igh erosion arc of this process is an' arc of similarcharacter, but in which the light is a loss and the erosion is broughtto themaximum.

The high'intensity are (including the high erosion arc) is anentirelydifierent electrical phenomena from thecommon arc. It comes intobeing as a sudden transition from the common are whencurrent densitiesat the anode face are employed which: exceed a critical value.- Thischange: in kind" of phenomenon is analogous to the ch'angewhich o'ccurs.at much lower current densities in the transition from a glow dischargeto a= common arc, which. transition. also I occurs suddenly when thecurrent density exceeds a critical point';

Th location of the critical point in both cases depends upon avariety offactors but in any given X casethe critical point is sharply shown andeasily not'too'felicitouswnd must not cause confusion between this newtype of arc and a, common are using a high current, since it is possibleto operate a common are with thousands of amperes without getting thehigh erosion effect, and on the other hand the high erosion effect maybe obtained with currents as low as amperes with very small electrodes.

The markedly new characteristics which come into being with high erosionarc, when the current density exceeds the critical point, include thefollowin In the normal arc, the crater of the anode, which is thehottest portion of the arc, reaches a temperature not greater than 3600"C. This temperature is not sufficient to cause more than a negligiblevaporization of the anode. The energy is dissipated predominately in theform of radiation, and the erosion of the anode that occurs in air isalmost wholly a slow oxidation of the solid incandescent carbon. In thenew are the temperature jumps to between 7000 and 10,000 C. At thattemperaturethe entire electrode including the carbon is vaporized atgreat speed, resulting in a superheated region of extremely brilliantgases immediately before the anode face which obscure the anode and inturn become an exceedingly luminous source of light. It is the highlight intensity of these vapors which first gave the new arc its name.

The result of this vaporization is also the emission from the anode of ahigh velocity jet of brilliantly luminous, highly ionized vapor whichshoots off out of the path of the current stream at a velocity of theorder of one sixth the velocity of sound, This brilliant jet may extendout as much as two feet from the electrode, al-

though the electrode spacing may not be more than an inch.

Another of the characteristics which come into being at the criticalpoint is a very sharp rise in the rate of erosion of the electrodes.With the common are the energy is largely converted into radiant energy,and an increase in energy input within that range does not make acorresponding increase in the rate of erosion. As soon as the criticalpoint of current density is reached, however, the very sharp bend occursupward in the curve between power input and erosion rate, and theerosion becomes substantially proportional to the power input.

There also occurs with these phenomena a marked change in thevolt-ampere curve of the discharge. In the normal arc range an increasein current is accompanied by a decrease in the voltage across the arc.Such an are therefore requires a ballast for operation. This is spokenof as a negative resistance characteristic.

Within the range of the high erosion are, however, the arc acts like anynormal resist.- ance, requiring additional voltage to send more current.This is commonly referred to as a positive resistance characteristic.This sudden change from negative to positive resistance characteristicis therefore another feature by which the high erosion arc can bedistinguished from the common are. i

In carrying out this process, the first step is the formation ofelectrodes composed-principally of the siliceous materials, withsuflicient carbon to render the electrode conductive usually with abonding agent. We have used electrodes comprising about 84% of thematerial to be treated with carbon and with a suitable binder. Thesematerials are thoroughly mixed and extruded to form green electrodes,preferably by a plunger extruder. Better electrodes are obtained if theextrusion is done under low gaseous pressure, since this permits theformation of dense electrodes without gas occlusion.

The electrodes after extrusion are baked at a temperature sufficient tocarbonize the volatile organic matter in the carbon source and causegood bonding of the materials to make the electrode homogeneous, havinglow resistivity. This temperature of baking will vary somewhat,depending in part upon the material that is being treated, and in partupon what particular form of carbon and what binding agent is employed.

We have found that when the bituminous coals of West Virginia,containing about 20% volatile carbonaceous matter, are used with sand,the preferable baking temperature is about 1500 C. for from ten tothirty minutes. This baking is conducted in such a manner as to avoidoxidation of the carbon of the electrode. We have conducted the bakingin graphite molds in which the electrodes were completely enclosed.

The electrodes are then used as the electrodes of a high erosion are,which may be burned in air. In this case the carbon content iscompletely removed from the product by oxidation. Such an arc produces ahigh evaporation of the electrode material. This vaporized material inthe case of sand condenses to liquid form at substantially 2200" C. toform a fog, which solidifies as solid particles at about 1500 C. Itpasses through these temperatures quickly and apparently withoutopportunity for the particles to coalesce or crystallize beforesolidification. It is a property of the high erosion arc that the vaporsissuing from it are extensively ionized. There is, therefore, a highdensity of electrical charges in the vapor stream, which are wellknownto be very effective as nuclei of condensation. These serve as centersfor the condensation of the cooling vapors, and are present in suchmultiplicity as to facilitate the condensation of the vapors into a finefume. This results in a. much higher degree of comminution than isotherwise possible, and the residual charge on the fine particlesgreatly facilitates their collection by electrostatic precipitation. Theproduct takes the form of very small spherical particles as seenwith theaid of the electron microscope. ranging in diameter from ten to onehundred milli-microns. By reason of their spherical form these particleshave no abrasive properties, and thereby provide a longer lastingrubber, and they are probably amorphous rather than crystalline in form.

Referring now to the drawing, the numeral [0 represents a chamber havingthree electrodes I I, I2 and I3 extending into it, but insulated fromit. This drawing is conventional only and any particular form of holdersfor the electrodes which will carry the high currents and provid for therapid rate of feed may be used.

The arc is then struck between these electrodes at the very high currentdensities required to produce the high erosion effect. This effect isthe rapid evaporation of the electrodes to produce a jet of vaporizedmaterial projected away from the electrodes. Theoretically, therefore,if direct current were available, it would be sufficient to put thematerial in the anode only. In practical operation, however, it issimpler to use alternating current because of its flexibility, so thatfor commercial reasons we prefer to use a three phase A. C. are havingall of the electrodes of the composition we have described.-

When the high erosion arc is maintained with alternating current, theelectrodes alternate as anodes, the resulting arc being in eifect asuccession of high intensity D. C. arcs of alternating polarity.

In practice by this process it has been found that with electrodeshaving a diameter of 16 mm. and a 16 kilowatt arc, with 200 amperes and80 volts, the fume may be produced with a power consumption of 3 kwh.per-pound of fume.

These vapors, which are luminous as they leave the arc crater, beingvisible as a tail flame projecting from a few inches to several feet(depending upon the size of the arc) beyond the crater, lose most oftheir energy by radiation to the walls of the arc chamber. Thus, despitethe high velocity with which the vapors are ejected from the arcto /2the velocity of sound-the temperature gradient is so rapid that by thetime the vapor has reached the tip of the tail flame the silica hasalready condensed to the solid state, and a steady stream of fume may beobserved issuing from the tip of the flame. In practice however, the gasstream, consisting of a mixture of air and CO2, must be cooled furtherin order to accomplish the collection at a reasonable temperature. Thisis shown diagrammatically as being performed by the heat exchanger IS.

The collection system is here shown in the form of a Cottrellprecipitator, having a casing l6 and an inner shell I! and an electrode18, all concentric with each other, the shell I! bein open at the bottomand having near its top an outlet 19 for gases and vapors of materialswhich are not condensed.

At the bottom of the precipitator is some control means 20 for closingoff the chamber or for permitting the withdrawal of the precipitatingcontents. These devices are standard apparatus and may take any standardform. The shell I! and the electrode 18 are oppositely chargedelectrically in the usual manner for Cottrell precipitators, and theshell I1 terminates short of the bottom of the casing 14. In this waythe gases going in from the connecting piece 20 with the fume,comprising chiefly air and oxides of carbon, may pass downwardly throughthe precipitator and upwardly through the shell I! and out through theoutlet IS. The fume evolved from the arc is electrically charged andthis facilitates their separation by the electrostatic "precipitator.

The size of the particles produced is in part determined by the speedwith which the gases are cooled to precipitation point. This in turnwill depend upon the length of the tail flame and, therefore, upon theoperating characteristics of the arc. However, the tail flame may beregulated independent of the arc operation by directing a stream of airat it so as to obtain virtually any size distributions desired over awide range.

The operation of the process will be clear from the above description.The are is established and maintained between the electrodes ll, l2 andI3, which are preferably formed of the mixture of the material to bevaporized and carbon substantially as specified.

Since certain changes may be made in carrying out the above processwithout departing from the scope of the invention, it is intended thatall matter contained in the above description shall be interpreted asillustrative and not in a limiting sense.

It is also to be understood that the following claims are intended tocover all of the generic and specific features of the invention hereindescribed, and all statements of the scope of the invention which as amatter of language might be said to fall therebetween.

What is claimed is:

1. The process of forming a fume from a refractory material, whichcomprises forming an electrode from a mixture of substantially 85% ofthe material with about 15% of a coking carbon and maintaining an arc inair with such electrode as anode at a current density at the anode facein excess of the transition point where the tail flame of luminousvapor, projected from the anode, appears and the resistancecharacteristic of the are changes from negative to positive, and thencondensing the vapors and col- "lecting the fume thus formed.

2. The process of forming a fume from silica, which comprises forming anelectrode from a mixture of substantially 85% of the silica with about15% of carbon and maintaining an arc in air with such electrode as anodeat a current density at the anode face in excess of the transi tionpoint where the tail flame of luminous vapor, projected from the anode,appears and the resistance characteristic of the are changes from:negative to positive, and then condensing the vapors and collecting thefume thus formed.

3. The process of producing a cement in fume form, which comprisesmixing the ground ingredients in proportions to form the cement, thenforming an electrode from a mixture of said ingredients with about 15%carbon added to render it conductive and then maintaining an arc in airwith such electrode as anode at a current density at the anode face inexcess of the transition i point where the tail flame of luminous vapor,

projected from the anode, appears and the resistance characteristic ofthe are changes from negative to positive, and then condensing thevapors and collecting the fume thus formed.

CHARLES SHEER. SAMUEL KORMAN.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 693,482 Acheson Feb. 18, 19021,054,372 Tone et al Feb. 25, 1913 1,418,528 Burgess June 6, 19221,650,894 Koehler Nov. 29, 1927 1,752,936 Austin et al. Apr. 1, 19301,982,012 Mingard Nov. 27, 1934 2,184,885 Muskat et al Dec. 26, 19392,306,184 Pechukas Dec. 22, 1942 2,399,68 McNabb May 71 1946

1. THE PROCESS OF FORMING A FUME FROM A REFRACTORY MATERIAL, WHICHCOMPRISES FORMING AN ELECTRODE FROM A MIXTURE OF SUBSTANTIALLY 85% OFTHE MATERIAL WITH ABOUT 15% OF A COKING CARBON AND MAINTAINING AN ARC INAIR WITH SUCH ELECTRODE AS ANODE AT A CURRENT DENSITY AT THE ANODE FACEIN EXCESS OF THE TRANSITION POINT WHERE THE TAIL FRAME OF LUMINOUSVAPOR, PROJECTED FROM THE ANODE, APPEARS AND THE RESISTANCECHARACTERISTIC OF THE ARC CHANGES FROM NEGATIVE TO POSITIVE, AND THENCONDENSING THE VAPORS AND COLLECTING THE FUME THUS FORMED.